It is almost certain that Matthew had seen Christ before and knew about him. He shows his gratitude by inviting him to come to his house to eat.
A new believer always witnesses or nearly always. He wants to reach his former friends with the gospel. What a tragedy that it fades and cools off. After years months, the new believer becomes indifferent to the need of others. We shouldn’t let it happen. Pray that God will keep alive a deep concern for others.
To what extent should we deliberately seek the company of men who are sinners for the purpose of winning them to Christ? Only if we can do so as a physician and not as a partner with them. Are we free to speak of the gospel? Is there something about the situation which is inherently evil? Are we dependent on their company? Are we being dragged down to their level? Are we calling them to leave an ungodly lifestyle?